Final answer:
The lung volume that cannot be measured by spirometry is the residual volume (RV), which is the air remaining in the lungs after maximal exhalation.
Step-by-step explanation:
The lung volume or capacity that cannot be measured by spirometry is the residual volume (RV). Residual volume is the amount of air that remains in the lungs after a maximal exhalation and cannot be completely exhaled from the lungs. This is because the lungs are never fully emptied; there is always some air left to prevent the lung tissues from sticking together, ensuring that the energy needed to re-inflate the lungs is not too great to overcome. It also plays a critical role in preventing large fluctuations in the concentrations of respiratory gases like oxygen and carbon dioxide. Since it is impossible to completely empty the lungs of air, residual volume can only be calculated rather than directly measured.